It seems clear I don't have the right files in the right places for GeomConverter to get the sampling fractions, but I don't really know what it needs. From what I've been able to dig up on the Internet, this simply isn't documented yet. I'll also mention that this command works when I use the sidloi3 detector.

I attached the detector's .zip file. While not all the files in SamplingFractions have the right filenames, it seems to me EcalEndcap should at least work.